1. Field of the Invention
The present invention relates to techniques for the measurement of fluid velocity and, in particular, to acoustic current meters for ocean deployment.
2. Description of the Prior Art
As the science of climatology and oceanography advance, observations at sea must be obtained more frequently and at shorter geographic intervals. One limiting factor is the high cost of conventional current meter designs and the associated costs for deployment and mooring systems.
Conventional current meters typically include relatively large, expensive electronics and battery packages and have limited operation lives. Current meters using mechanical sensors, such as rotors or impellers, are vulnerable to
damage and are easily stalled by marine fouling or flotsam of various kinds.
Conventional acoustic current meters using two orthogonal pairs of transducers and an acoustic mirror have poor vertical and horizontal cosine response due to flow interaction with the mirror and struts used to carry mooring tension around the pressure housing. Conventional acoustic current meters transmit pulses or continuous wave--or CW--bursts simultaneously in opposite directions and then compare the difference in arrival times or the phase difference between the received signals to determine current velocity. Such instruments require two receivers and are subject to errors due to changes in phase shift or time delay differences between the receivers.
Such conventional instruments require gimballed compasses, such as magnetometers or mechanical compasses with optical readouts, which tend to be fragile, expensive and of limited accuracy. The direction sensors in conventional instruments used either gimballed compass cards with optical readouts or gimballed 2 axis fluxgate magnetometers. The designs including compass cards are fragile, expensive and do not have good dynamic response because of the inertia of the cards and the low magnetic torque inherent in such compass cards. The gimballed fluxgate designs require jewel bearings to minimize errors due to imperfect leveling caused by bearing stickiness. As a result, such designs required enclosure of the gimballed fluxgate in an oil filled chamber to provide damping. What is needed is an improved acoustic current meter design which is less expensive to produce, deploy and moor and does not suffer from the limitations of known designs.